I've just had a look at this paper too:
They noted not much research into impact of health conditions on personality... they must have not searched for CFS stuff! We're so lucky.
Dolphin already highlighted some of this quote, but I'm including the bit on the expected decrease in emotional stability (increase in neuroticism):
Based on previous studies of chronic diseases and
quality of life (Stanton et al. 2007; de Ridder et al.
2008) we hypothesized that the onset of a disease
would be associated with a decrease in emotional stability
due to psychological distress, disability, and limitations
to daily activities caused by the disease. We
also hypothesized a decrease in extraversion, because
several chronic diseases cause fatigue and loss of energy
(de Ridder et al. 2008) that potentially decrease
features of extraversion, such as positive mood, activity,
and approach-motivated behaviour.
Dolphin already highlighted this, but I'll do so again, as it seems really relevant to what had always seemed to be poor arguments supporting the primarily psychosocial nature of MUS, with the claim that the association between lots of symptoms and emotional distress indicated that psychological factors play a causative (or 'perpetuating') role in symptoms:
The associations followed a dose– response pattern, with each chronic condition strengthening the association in a linear fashion.
Seems pretty unsurprising to me that lots of symptoms takes more of an emotional toll than just one.
Also - the effect size didn't need to be large for them to be able to report significant results.
It might have been interesting to see if they'd found the certain personality traits at the first time point were associated with an increased risk of going on to develop particular health problems.
One might then expect that ME/CFS, which often is like having several impairments in terms of the number of symptoms and in the impairment, would have big effect on personality scores.
Also - ME/CFS is unusual because of the prejudices/lack of support/quackery that often comes with it. It's really difficult to control for that, yet these seem to be exactly the sort of things likely to effect 'personality'.